The fluorescent lamp is a gas discharge tube that is used for lighting purposes. Generally, the inner surface of the wall of the tube is coated with light-emitting substances—usually fluorescent or phosphorescent metallic salts, and the tube is filled with mercury vapor at extremely low pressure and has filaments at each end of the tube. The light of the fluorescent lamp is not produced by an incandescent body (such as the filament of an ordinary electric lamp), but is emitted as a result of the excitation of atoms (namely, those of the mercury vapor and the fluorescent coating) and is extremely economical. The electrons ejected from the cathode filaments collide with the mercury atoms of the vapor and cause the mercury atoms to emit radiation which consists for the most part of ultraviolet rays, which are invisible. The ultraviolet light strikes the fluorescent substance with which the wall of the tube is coated and, depending upon the coating, may cause the substance to emit radiation with a longer wavelength in the visible rage of the spectrum—i.e., the coating may transform the invisible rays into visible light.
The conventional fluorescent lamp has to be operated with a choke (normally referred to as a ballast), which prevents a harmful rise in voltage and serves to ignite the lamp. For this purpose a starter circuit comprising a small auxiliary glow lamp provided with a thermal contact is usually connected in parallel with the main lamp. When the current is switched on, the glow lamp first lights up (the thermal contact is now open). This causes the contact to warm up and close, with the result that the glow lamp is short-circuited and the cathodes of the main lamp receive the full current. The thermal contact then cools and breaks, providing a voltage surge which is high enough to initiate the discharge in the fluorescent lamp itself. Because it is bypassed by the main lamp, the small auxiliary glow lamp then ceases to function.
Ultraviolet fluorescent lamps are often used in museum and other displays where powerful lighting is required to properly irradiate and display fluorescent mineral specimens. Fluorescent lamps are used with special ultraviolet filters that transmit the ultraviolet light and absorb the visible light that is generated by the lamps. However, most ultraviolet fluorescent lamps in use today have short useful life spans, and it is most commonly due to the inability of conventional drive circuits with conventional starter circuits to handle the high number of on-off cycles necessary in such a museum or other display. Another drawback of the currently available lamps is that no manufacturer of commercial ultraviolet lights uses a high output lamp.
Many prior inventors, such as Ewest and Yamamoto, have recognized the need to improve the starting and operating efficiency of fluorescent lamps. However, these prior art patents are directed simply to alternative designs of starter circuits.
Ewest, in U.S. Pat. No. 1,961,749, discloses a gaseous electric discharge device which uses an auxiliary electrode in addition to the main electrode at one end of the lamp tube. This auxiliary electrode in proximity to the main electrode serves as a starter “glow lamp” component for the main lamp. Ewest's device also uses a high frequency apparatus to ionize the gas within the tube.
Yamamoto et al., in U.S. Pat. No. 5,107,183, disclose a fluorescent lamp which also uses a special arrangement of electrodes at one end or both ends of the lamp to constitute a starter “glow lamp” component. Both Ewest's device and Yamamoto's device, because they use the same current supply to start the lamp as well as to maintain operation of the lamp, would have the same short lifespans of other extant devices.